Re: [Discuss-gnuradio] low frequency oscope

[ematlis]

Yes, it's 10 Hz.  And a 50 Hz high-pass; I originally didn't have that in 
place, thinking it would filter off the 4 Hz, but in fact it doesn't seem 
to, although it does remove most of the DC.  Could this problem be an 
aliasing problem?  Or could it be related to the frequency resolution of 
the local oscillator?

thanks,
eric

On Thu, 13 Sep 2007, Eric Blossom wrote:

> On Thu, Sep 13, 2007 at 12:53:35PM -0400, address@hidden wrote:
> > Hi all-
> >
> > I'm trying to understand the behavior of the USRP/LFRX when used as a low
> > frequency oscilloscope.  I am trying to measure a AM modulated waveform
> > with carrier frequency 34 kHz, modulated at 4 Hz.  The decimation in the
> > FPGA is 250, with a 10 Hz low-pass fir filter with a decimation factor of
>
> Do you really mean 10 Hz low-pass filter, or is this a typo?
>
> > 64 giving an effective sampling rate of 4 kHz, and a high-pass filter at
> > 50 Hz to remove the 0 Hz (DC) component created by the demodulation.  I am
> > using the std_4rx_0tx.rbf firmware to enable 4 channel reception.
>
> > Contrary to my expectations, I can tune to USRP off the 34 kHz and still
> > pickup the 4 Hz; in fact, it's better if I do tune away from 34 kHz!  I
> > can tune anywhere between 0 and 34.98 and I get a nice waveform.  I I tune
> > to exactly 34 kHz, I get a low frequency modulation at roughly 1.67 Hz on
> > top of the 4 Hz I'm trying to measure.  Here's a snippet of my
> > (psuedo) code for the first channel:
> >
> > usrp --> dinterleaver --> (dinterleaver_ch0)magblock --> lpf --> hpf -->
> > scope.
> >
> > Any thoughts?
> >
> > thanks!
> > eric